Duct rated cable is actually called plenum cabling. It's the same CAT5, 5E, or 6 but has a different exterior that's heat rated. Most riser cable is only rated to 60 degrees C, which you could exceed inside a furnace duct close to your furnace.

You can get shielded and unshielded. My local electrical distributor is selling CAT5E riser for $79 per 1000' box, and unshielded Plenum for $185/box.